The present invention relates to methods for controlling the quality of an industrial process, comprising the steps of:
making available one or more reference signals relating to the industrial process
acquiring one or more real signals indicating the quality of said industrial process,
comparing said one or more reference signals to said one or more real signals to identify defects of said industrial process.
Monitoring defects in industrial processes is assuming a growing economic importance due to its impact in the analysis of the quality of industrial products. The ability to obtain an assessment of the quality of the industrial process on line and automatically has many advantages, both in economic terms and in terms of process velocity. Therefore, the desirable characteristics of the system are:                on line and real time processing;        ability to recognise the main production defects with accuracy.        
Currently, the problem of recognising the quality of an industrial process, and thus of identifying any defects, takes place through an off-line inspection conducted by experts, or with automatic methods which, through sensors, identify only some of the aforementioned defects, in a manner that is not satisfactory and that is also sensitive to the different settings of the machine.
Methods and systems for controlling the quality of industrial processes are known, for instance applied to the on-line monitoring of the laser welding process, in particular in the case of metal plate welding. The controlling system is able to assess the presence of porosities in the welded area or, in the case of butt-welded thin metal plates, the presence of defects due to the superposition or to the disjunction of the metal plates.
Such systems in use base quality control on a comparison between the signals obtained during the process and one or more predetermined reference signals, indicative of a high quality weld. Such reference signals, usually in a variable number between two and ten, are predetermined starting from multiple samples of high quality welds. This manner of proceeding implies the presence of an experienced operator able to certify the quality of the weld at the moment of the creation of the reference signals, entails time wastage and at times also material wastage (which is used to obtain the samples needed to obtain the reference signals). It would therefore be necessary, given a similar procedure, onerous in itself in terms of time and cost, for the subsequent procedure of comparison with the reference signal to be able to operate rapidly, in real time and at low cost, which does not take place in currently known systems.
Also known, for example from the European patent application EP-A-1275464 by the same Applicant, are methods that avoid use of the reference by means of procedures of statistical analysis of the radiation emitted by the welding spot; however, these methods allow only a very approximate detection of any defects.